These studies will explore factors that limit the exchange of oxygen, carbon dioxide, and water across the placenta and their distribution throughout the fetus by the circulation. We propose to study the role of maternal and fetal blood flows in limiting placental exchange and the mechanisms regulating these flows. More specifically we will explore the effect of hydrostatic and osmotic forces on placental water transfer, the effect of water transfer on fetal blood volume, the compliance of the umbilical circulation, the possible role of cytochromes in facilitating O2 diffusion, and the dependence of fetal O2 consumption on arterial venous PO2. In addition to these experimental studies, mathematical models will be developed. We seek to increase the understanding of fetal oxygenation and prevent developmental problems associated with inadequate placental transfer.